Alfred Stieglitz (January 1, 1864 – July 13, 1946) was an American-born photographer who was instrumental over his fifty-year career in making photography an acceptable art form alongside painting and sculpture. Many of his photographs are known for appearing like those other art forms, and he is also known for his marriage to painter Georgia O'Keeffe.

Stieglitz was born the eldest of six children in Hoboken, New Jersey and raised in a brownstone on Manhattan's Upper East Side. His father moved with his family to Germany in 1881. The next year, Stieglitz began studying mechanical engineering at the Technische Hochschule in Berlin and soon switched to photography. Traveling through the European countryside with his camera, he took many photographs of peasants working on the Dutch seacoast and undisturbed nature within Germany's Black Forest and won prizes and attention throughout Europe in the 1880s.

Throughout his life, Stieglitz was infatuated with younger women. He married Emmeline Obermeyer in 1893, after he returned to New York, and they had one child, Kitty, in 1898. Allowances from Emmeline's father and his own enabled Stieglitz to not have to work for a living. From 1893 to 1896, Stieglitz was editor of American Amateur Photographer magazine; however, his editorial style proved to be brusque, autocratic, and alienating to many subscribers. After being forced to resign, Stieglitz turned to the New York Camera Club (which was later renamed The Camera Club of New York and is in existence to this day) and retooled its newsletter into a serious art periodical known as Camera Notes. He announced that every published image would be a picture, not a photograph - a statement that allowed Stieglitz to determine which was which by his scientific method.

Big camera clubs that were the vogue in America then did not satisfy him; in 1902 he organized an invitation-only group he dubbed the Photo-Secession to force the art world to recognize photography "as a distinctive medium of individual expression." Among its members were Edward Steichen, Gertrude Kasebier, Clarence White, and Alvin Langdon Coburn. Photo-Secession held its own exhibitions and published Camera Work, a pre-eminent quarterly photographic journal, until 1917.

From 1905 to 1917, Stieglitz managed the Little Galleries of the Photo-Secession at 291 Fifth Avenue (which came to be known as the 291). In 1910, Stieglitz was invited to organize a show at Buffalo's Albright-Knox Art Gallery that set attendance records. He was insistent that "photographs look like photographs," so that their realism would allow painting to become more abstract. This shift to abstract art mystified Camera Work subscribers and the viewing public.

Stieglitz divorced his wife Emmeline in 1918, soon after she threw him out of their house when she came home and found him photographing Georgia O'Keeffe, whom he moved in with shortly thereafter. They married in 1924 and were both successful, he in photography (he would take hundreds of pictures of her throughout his life), she as an artist who had received notoriety from Stieglitz at the 291 years before. However, their marriage became strained as she had to care more for Stieglitz's health due to a prevailing heart condition and his hypochondria. By the 1930s, she would spend six months out of the year away from him in New Mexico.

In the 1930s, Stieglitz took a series of photographs, some nude, of heiress Dorothy Norman, who became in O'Keeffe's mind a serious rival for Stieglitz's affections. Both these photos and those of O'Keeffe are often recognized as the first photographs to recognize the potential of isolated parts of the human body. In these years, he also presided over two non-commercial New York City galleries, The Intimate Gallery and An American Place.

Stieglitz's camera work ended in 1937 due to heart disease. Over the last ten years of his life, he summered at Lake George, New York and worked in a shed he had converted into a darkroom and wintered with O'Keeffe in Manhattan's Shelton, the first skyscraper hotel in that city. He died in 1946 at 82, still a staunch supporter of O'Keeffe, and she of him.